I've got a soft spot for hitchhiker crabs so I'm probably unknowingly trying to find an excuse here, but do you often stir up the gravel? There is a possibility this will have released some sort of toxin or otherwise just created anoxic conditions in the tank, and the clownfish was fatally affected by these. Do you have other fish?
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-Quinn
Man, n. ...His chief occupation is extermination of other animals and his own species, which, however, multiplies with such insistent rapidity as to infest the whole habitable earth, and Canada. - A. Bierce, Devil's Dictionary, 1906
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